--- 1/draft-ietf-v6ops-pmtud-ecmp-problem-00.txt 2015-05-19 23:17:11.027234421 -0700
+++ 2/draft-ietf-v6ops-pmtud-ecmp-problem-01.txt 2015-05-19 23:17:11.055235095 -0700
@@ -1,46 +1,46 @@
v6ops M. Byerly
Internet-Draft Fastly
Intended status: Informational M. Hite
-Expires: September 2, 2015 Evernote
+Expires: November 20, 2015 Evernote
J. Jaeggli
Fastly
- March 1, 2015
+ May 19, 2015
Close encounters of the ICMP type 2 kind (near misses with ICMPv6 PTB)
- draft-ietf-v6ops-pmtud-ecmp-problem-00
+ draft-ietf-v6ops-pmtud-ecmp-problem-01
Abstract
This document calls attention to the problem of delivering ICMPv6
type 2 "Packet Too Big" (PTB) messages to the intended destination in
- ECMP load balanced, or anycast network architectures. It discusses
+ ECMP load balanced or anycast network architectures. It discusses
operational mitigations that can be employed to address this class of
failure.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
- This Internet-Draft will expire on September 2, 2015.
+ This Internet-Draft will expire on November 20, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
@@ -50,70 +50,71 @@
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Mitigation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Alternatives . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Implementation . . . . . . . . . . . . . . . . . . . . . 5
- 4. Improvements . . . . . . . . . . . . . . . . . . . . . . . . 6
+ 3.2.1. Alternatives . . . . . . . . . . . . . . . . . . . . 6
+ 4. Improvements . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
- 8. Informative References . . . . . . . . . . . . . . . . . . . 7
- Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
+ 8. Informative References . . . . . . . . . . . . . . . . . . . 8
+ Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Operators of popular Internet services face complex challenges
associated with scaling their infrastructure. One approach is to
utilize equal-cost multi-path (ECMP) routing to perform stateless
distribution of incoming TCP or UDP sessions to multiple servers or
to middle boxes such as load balancers. Distribution of traffic in
this manner presents a problem when dealing with ICMP signaling.
Specifically, an ICMP error is not guaranteed to hash via ECMP to the
same destination as its corresponding TCP or UDP session. A case
where this is particularly problematic operationally is path MTU
discovery (PMTUD).
2. Problem
A common application for stateless load balancing of TCP or UDP flows
is to perform an initial subdivision of flows in front of a stateful
- load balancer tier or multiple servers, so that the workload becomes
+ load balancer tier or multiple servers so that the workload becomes
divided into manageable fractions of the total number of flows. The
flow division is performed using ECMP forwarding and a stateless but
sticky algorithm for hashing across the available paths. This
nexthop selection for the purposes of flow distribution is a
- constrained form of anycast topology, where all anycast destinations
+ constrained form of anycast topology where all anycast destinations
are equidistant from the upstream router responsible for making the
last next-hop forwarding decision before the flow arrives on the
destination device. In this approach, the hash is performed across
some set of available protocol headers. Typically, these headers may
include all or a subset of (IPv6)Flow-Label, IP-source, IP-
destination, protocol, source-port, destination-port and potentially
others such as ingress interface.
A problem common to this approach of distribution through hashing is
impact on path MTU discovery. An ICMPv6 type 2 PTB message generated
- on an intermediate device for a packet sent from an a server that is
- part of an ECMP load balanced service to a client, will have the
- load-balanced anycast address as the destination and would be
+ on an intermediate device for a packet sent from a server that is
+ part of an ECMP load balanced service to a client will have the load
+ balanced anycast address as the destination and hence will be
statelessly load balanced to one of the servers. While the ICMPv6
PTB message contains as much of the packet that could not be
- forwarded as possible, the payload headers are not considered into
- the forwarding decision and are ignored. Because the PTB message is
- not identifiable as part of the original flow by the IP or upper
- layer packet headers the results of the ICMPv6 ECMP hash are unlikely
- to be hashed to the same nexthop as packets matching TCP or UDP ECMP
+ forwarded as possible, the payload headers are not considered in the
+ forwarding decision and are ignored. Because the PTB message is not
+ identifiable as part of the original flow by the IP or upper layer
+ packet headers, the results of the ICMPv6 ECMP hash are unlikely to
+ be hashed to the same nexthop as packets matching TCP or UDP ECMP
hash.
An example packet flow and topology follow.
ptb -> router ecmp -> nexthop L4/L7 load balancer -> destination
router --> load balancer 1 --->
\\--> load balancer 2 ---> load-balanced service
\--> load balancer N --->
@@ -122,93 +123,93 @@
The router ECMP decision is used because it is part of the forwarding
architecture, can be performed at line rate, and does not depend on
shared state or coordination across a distributed forwarding system
which may include multiple linecards or routers. The ECMP routing
decision is deterministic with respect to packets having the same
computed hash.
Atypical case where ICMPv6 PTB messages are received at the load
balancer is a case where the path MTU from the client to the load
balancer is limited by a tunnel in which the client itself is not
- aware of. In the common case of a TCP flow where TLS is employed,
- the first packet sent from the server that is likely to exceed a
- tunnel MTU lower than that specified by the MSS on the client and the
- load balancer/server is the TLS ServerHello and certificate.
+ aware of.
Direct experience says that the frequency of PTB messages is small
compared to total flows. One possible conclusion being that tunneled
- IPv6 deployments that cannot carry 1500 mtu packets are relatively
+ IPv6 deployments that cannot carry 1500 MTU packets are relatively
rare. Techniques employed by clients such as happy-eyeballs may
actually contribute some amelioration to the IPv6 client experience
by preferring IPv4 in cases that might be identified as failures.
Still, the expectation of operators is that PMTUD should work and
that unnecessary breakage of client traffic should be avoided.
A final observation regarding server tuning is that it is not always
possible even if it is potentially desirable to be able to
- independently set the TCP MSS for different address families on end-
- systems.
+ independently set the TCP MSS for different address families on some
+ end-systems. On Linux platforms, advmss may be set on a per route
+ basis for selected destinations in cases where discrimination by
+ route is possible.
- The problem as described does also impact IPv4; however, the ability
- to fragment on wire at tunnel ingress points and the relative rarity
- of sub-1500 byte MTUs that are not coupled to changes in client
- behavior (for example, endpoint VPN clients set the tunnel interface
- MTU accordingly for performance reasons) makes the problem
- sufficiently rare that some existing deployments simply choose to
- ignore it.
+ The problem as described does also impact IPv4; however
+ implementation of RFC 4821 [RFC4821] TCP MTU probing, the ability to
+ fragment on wire at tunnel ingress points and the relative rarity of
+ sub-1500 byte MTUs that are not coupled to changes in client behavior
+ (for example, endpoint VPN clients set the tunnel interface MTU
+ accordingly for performance reasons) makes the problem sufficiently
+ rare that some existing deployments have choosen to ignore it.
3. Mitigation
Mitigation of the potential for PTB messages to be mis-delivered
involves ensuring that an ICMPv6 error message is distributed to the
same anycast server responsible for the flow for which the error is
- generated. Ideally Mitigation could be done by the mechanism hosts
+ generated. Ideally, mitigation could be done by the mechanism hosts
use to identify the flow, by looking into the payload of the ICMPv6
message (to determine which TCP flow it was associated with) before
making a forwarding decision. Because the encapsulated IP header
- occurs at a fixed offset in the icmp message it is not outside the
+ occurs at a fixed offset in the ICMP message it is not outside the
realm of possibility that routers with sufficient header processing
capability could parse that far into the payload. Employing a
mediation device that handles the parsing and distribution of PTB
messages after policy routing or on each load-balancer/server is a
possibility.
Another mitigation approach is predicated upon distributing the PTB
message to all anycast servers under the assumption that the one for
which the message was intended will be able to match it to the flow
- and update the route cache with the new MTU, devices not able to
- match the flow will discard these packets. Such distribution has
+ and update the route cache with the new MTU and that devices not able
+ to match the flow will discard these packets. Such distribution has
potentially significant implications for resource consumption and the
potential for self-inflicted denial-of-service if not carefully
- employed. Fortunately, in real-world-deployment we have observed
- that, the number of flows for which this problem occurs is relatively
+ employed. Fortunately, in real-world deployments we have observed
+ that the number of flows for which this problem occurs is relatively
small (example, 10 or fewer pps on 1Gb/s or more worth of https
traffic) and sensible ingress rate limiters which will discard
excessive message volume can be applied to protect even very large
anycast server tiers with the potential for fallout only under
circumstances of deliberate duress.
3.1. Alternatives
As an alternative, it may be appropriate to lower the TCP MSS to 1220
in order to accommodate 1280 byte MTU. We consider this undesirable
as hosts may not be able to independently set TCP MSS by address-
family thereby impacting IPv4, or alternatively that it relies on a
middle-box to clamp the MSS independently from the end-systems.
3.2. Implementation
1. Filter-based-forwarding matches next-header ICMPv6 type-2 and
matches a next-hop on a particular subnet directly attached to
both border routers. The filter is policed to reasonable limits
- (we chose 1000pps).
+ (we chose 1000pps more conservative rates might be required in
+ other imlementations).
2. Filter is applied on input side of all external interfaces
3. A proxy located at the next-hop forwards ICMPv6 type-2 packets
received at the next-hop to an Ethernet broadcast address
(example ff:ff:ff:ff:ff:ff) on all specified subnets. This was
necessitated by router inability (in IPv6) to forward the same
packet to multiple unicast next-hops.
4. Anycast servers receive the PTB error and process packet as
@@ -238,48 +239,71 @@
if __name__ == '__main__':
main()
This example script listens on all interfaces for IPv6 PTB errors
being forwarded using filter-based-forwarding. It removes the
existing Ethernet source and rewrites a new Ethernet destination of
the Ethernet broadcast address. It then sends the resulting frame
out the p2p1 and p2p2 interfaces where our anycast servers reside.
+3.2.1. Alternatives
+
Alternatively, network designs in which a common layer2 network
- exists could rewrite the destination on the end system, for example
- in using iptables before forwarding the packet back to the network
- containing all of the server or load balancer interfaces.
+ exists on the ECMP hop could distribute the proxy onto the end
+ systems, eleminating the need for policy routing. They could then
+ rewrite the destination -- for example, using iptables before
+ forwarding the packet back to the network containing all of the
+ server or load balancer interfaces. This implmentation can be done
+ entirely within the Linux iptables firewall. Because of the
+ distributed nature of the filter, more conservative rate limits are
+ required than when a global rate limit can be employed.
+
+ An example ip6tables / nftables rule to match icmp6 traffic, not
+ match broadcast traffic, impose a rate limit of 10 pps, and pass to a
+ target destination would resemble:
+
+ ip6tables -I INPUT -i lo -p icmpv6 -m icmpv6 --icmpv6-type 2/0 \
+ -m pkttype ! --pkt-type broadcast -m limit --limit 10/second \
+ -j TEE 2001:DB8::1
+
+ As with the scapy example, once the destination has been rewritten
+ from a hardcoded ND entry to an Ethernet broadcast address -- in this
+ case to an IPv6 documentation address -- the traffic will be
+ reflected to all the hosts on the subnet.
4. Improvements
There are several ways that improvements could be made to the
situation with respect to ECMP load balancing of ICMPv6 PTB.
1. Routers with sufficient capacity within the lookup process could
parse all the way through the L3 or L4 header in the ICMPv6
payload beginning at bit offset 32 of the ICMP header. By
reordering the elements of the hash to match the inward direction
of the flow, the PTB error could be directed to the same next-hop
as the incoming packets in the flow.
- 2. The FIB could be programmed with a multicast distribution tree
- that included all of the necessary next-hops.
+ 2. The FIB (Forwarding Information Base) on the router could be
+ programmed with a multicast distribution tree that included all
+ of the necessary next-hops.
3. Ubiquitous implementation of RFC 4821 [RFC4821] Packetization
Layer Path MTU Discovery would probably go a long way towards
reducing dependence on ICMPv6 PTB.
5. Acknowledgements
- The authors would like to thank Mark Andrews, Brian Carpenter, Nick
- Hilliard and Ray Hunter, for review.
+ The authors would like to thank Marak Majkowsiki for contributing
+ text, examples, and a very close review. The authors would like to
+ thank Mark Andrews, Brian Carpenter, Nick Hilliard and Ray Hunter,
+ for review.
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
The employed mitigation has the potential to greatly amplify the
impact of a deliberately malicious sending of ICMPv6 PTB messages.
Sensible ingress rate limiting can reduce the potential for impact;
@@ -297,24 +321,25 @@
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
Authors' Addresses
Matt Byerly
Fastly
Kapolei, HI
US
- Email: mbyerly@zynga.com
+ Email: suckawha@gmail.com
Matt Hite
Evernote
Redwood City, CA
US
Email: mhite@hotmail.com
+
Joel Jaeggli
Fastly
Mountain View, CA
US
Email: joelja@gmail.com